A tall structure such as a wind turbine is equipped with a lightning protection system (LPS) which serves to receive a stroke at one or more specific sites or receptors, and to divert the ensuing electrical current to ground during a lightning strike. The blades of a wind turbine are particularly vulnerable to lightning strikes on account of their length. The LPS of a wind turbine therefore generally includes a lightning conductor for each blade, arranged in the interior of the blade and extending along its length from base to tip. The blades are generally mounted to a hub or spinner, and the combined arrangement rotates relative to the stationary nacelle or canopy. The rotating assembly is connected to the stationary nacelle by means of a bearing. To electrically connect the rotating hub and blade arrangement to the relatively stationary nacelle, each LPS conductor of a blade terminates in a sliding brush that effects the electrical connection to the stationary conductive ring arranged in the hub. The conductive ring in turn is electrically connected to ground, so that, in the event of a lightning strike to a receptor of a blade, the electrical current sees a direct path along the blade lightning conductor and across the brush to ground.
A problem with this kind of lightning protection system arises on account of the nature of the electrical system defined by the moving and stationary lighting conductors, the interface between them, the outside environment, and the environment inside the hub. As the long blades move through the air, static electricity will build up on the LPS conductors and therefore also on the brushes, particularly when the air is very dry. The level of static electricity build-up on a long LPS down-conductor can be extreme in the presence of an elevated electrical field potential, for example during a storm. Furthermore, a powerful radio transmitter in the neighbourhood can induce high frequency currents in an LPS down-conductor, which effectively acts as an antenna. In an ideal LPS environment, static electricity build-up on a down-conductor or induced currents in a down-conductor would not be a problem, since the purpose of the sliding brushes is to provide a discharge path to ground. However, the environment inside the hub is not ideal, since contamination in the form of oil, grease and/or particles detracts from the function of the sliding brushes. Lubricant can escape or leak from the various drives, motors and hydraulic components in the hub, and can be deposited as a thin film over any surface in the hub interior. Even if such components are well sealed, it is basically unavoidable that some quantity of grease or oil will escape over the course of time. If a thin film of grease is deposited along the path travelled by the sliding brushes, the grease film effectively acts as an insulator between the blade conductor and electrical ground. As a result, a voltage will build up between the brush and the thin film of grease. Eventually, the voltage will reach the level of a break-down voltage for that thin film, at which point the insulator collapses and conducts, allowing the built-up static electricity to discharge to ground. After discharging, static will start to build up again, and the build-up/breakdown cycle will repeat indefinitely.
The breakdown discharge spark is associated with emission of wide-band electromagnetic radiation. The LPS of the wind turbine therefore unintentionally acts as a source of electromagnetic noise. The level of electromagnetic noise can reach levels that compromise adherence to electromagnetic compatibility (EMC) limits laid down by various standards.